Anas bin Malik reported,

He saw some phlegm on the qiblah wall of the mosque, and it troubled him so much that the displeasure could be seen on his face. He scraped it off and said, "When one of you," or, "when a person stands to pray, he is conversing privately with his Lord ," or, "his Lord is between him and the qiblah. So if he must spit, let him spit to his left or under his foot." Then he gestured like this, as though spitting into his garment.He said, We used to ask Humayd about this, and he would say, "Glory be to Allah! Who do you think it was?" meaning the Prophet ﷺ, without telling us anything more.

حدثنا محمد بن عبد الله بن المثنى حدثنا حميد

عن أنس قال

رأى نخامة في قبلة المسجد فشق عليه حتى عرف ذاك في وجهه فحكه وقال إن أحدكم أو المرء إذا قام إلى الصلاة فإنه يناجي ربه عز وجل أو ربه بينه وبين القبلة فليبزق إذا بزق عن يساره أو تحت قدمه وأومأ هكذا كأنه في ثوبه قال وكنا نقول لحميد فيقول سبحان الله من هو يعني النبي ﷺ ولا يزيدنا عليه